The invention refers to a circuitry for telephone systems for the transmission of alternating current signals from a subsequent telephone system to a supplying exchange, whereby a signal transformer is provided with a by-pass for the direct current in both lines of the supplying exchange.
Signal transformers are typically used in telephone systems for voice signal transmission from subordinate telephone systems such as PBX, PABX or other subscriber systems to a superior supplying exchange such as a central office. These signal transformers are employed for the voice signal as well as for the direct current provided by the supplying exchange which is necessary for the transmission of communication signalling. Due to the qualitative requirements of signal transmission the transformers are comparatively large, heavy and expensive. By series connection with a capacitor it is possible to keep the direct current of the supplying exchange away from the transformer and thus to avoid DC magnetization of the transformer. This allows the size of the transformer to be reduced. However the size reduction is limited because the total analog transmission signal energy still must be transmitted to the supplying exchange or central office.
Moreover, conventional circuitry is required to provide a by-pass for the transformer and the capacitor to maintain the direct current flow in the line of the superior exchange. For this purpose a coil with a relatively high inductance is used which causes additional circuit complexity. Another solution for the current path is electronic circuitry with a transistor, which, if appropriately equipped forms an impedance making direct current flow possible. This transistor circuit requires considerable complexity and does not enable a substantial reduction of transformer dimensions.